1. Field of the Invention
This invention relates to jet powered watercraft, especially personal watercraft (“PWCs”). More specifically, the invention concerns the jet propulsion system of the watercraft. In particular, the invention is directed to controlling thrust.
2. Description of Related Art
Jet powered watercraft have become very popular in recent years for recreational use and for use as transportation in coastal communities. The jet power offers high performance, which improves acceleration, handling and shallow water operation. Accordingly, PWCs and jet boats, which typically employ jet propulsion, have become common place, especially in resort areas. As use of PWCs and jet boats has increased, operators expect better performance, including greater operational efficiency, higher top speed, faster acceleration, and improved control.
FIG. 12 illustrates a prior art jet propulsion system 600 disposed within a hull 612 of a watercraft, of which only a portion is shown in broken lines. As shown, water enters the tunnel 614 of the jet propulsion system 600 through an inlet grate 616 and intake ramp 618. From the water intake ramp 618, water enters into the jet pump (or jet propulsion unit) 624, which is supported above a ride plate 625. The jet pump 624 includes an impeller 626 that is rotationally coupled to an engine by one or more shafts 632, such as a drive shaft and/or an impeller shaft. The rotation of the impeller 626 pressurizes the water, which then moves over a stator 628. The stator 628 is attached to a jet propulsion unit housing 636 and decreases the rotational motion of the water so that almost all the energy given to the water is used for thrust, as opposed to swirling the water. As shown, the impeller 626 and the stator 628 are both disposed within the jet propulsion unit housing 636 or pump housing. However, it is also known to position the stator 628 at a position outside of the housing 636 downstream of the housing 636.
Once the water leaves the jet pump 624, it accelerates further through a venturi 640. In this prior art jet propulsion system 600, the venturi 640 is disposed at the rearward end of the housing 636. As shown, the venturi 640 is attached to the housing 636 and defines the outlet from the housing 636. A steering nozzle 644 is pivotally attached to the venturi 640 so as to pivot about a vertical axis.
A water passage 646, through which water passes from left to right, is illustrated in FIG. 15. Moving from left to right in this illustration, which is upstream to downstream, the water passage 646 is defined by the inlet grate 616, the water intake ramp 618, the jet pump 624, the venturi 640, and the steering nozzle 644.
Because a discharge opening 640a of the venturi 640 is smaller than its inlet opening 640b, the water is accelerated through the venturi 640. As would be appreciated by one of ordinary skill in fluid dynamics, the size (cross-sectional area and/or shape) of the discharge opening 640a of the venturi 640 relative to its inlet opening 640b and the remaining water passage 646 determines several key operating parameters of the jet propulsion system 600. If the discharge opening 640a is relatively small, the jet propulsion system 600 pushes a smaller volume of faster moving water out through the discharge opening 640a. The small discharge opening 640a could results in a higher top speed, but a slower acceleration for the watercraft. Conversely, if the discharge opening 640a is relatively larger, the jet propulsion system 600 pushes a larger volume of slower moving water out through the discharge opening 610 to propel the system 600. The larger discharge opening 640a could therefore results in a slower top speed, but a faster acceleration. Furthermore, an optimized intermediately-sized discharge opening 640a would give optimum fuel economy at a given speed, yet would not result in top speed or high acceleration.
Because conventional venturis 640 have fixed-area discharge openings, venturi designers must balance the competing goals of maximizing top speed, acceleration, and fuel efficiency when designing venturis 640. Further, the only way to control the thrust in a fixed-area venturi is to control the engine speed.
One way of addressing these problems is to vary the flow volume through the venturi. This has been attempted in various ways in the prior art. To maximize top speed, acceleration, and fuel efficiency using a single venturi, variable venturis have been developed that vary the cross-sectional area of the discharge opening during operation of the jet propulsion system and watercraft. Such variable venturis allow watercraft to dynamically, selectively maximize top speed, acceleration, or fuel efficiency at any given instant by changing the cross-sectional area of the discharge opening of the venturi. Some examples of various methods of dynamically altering the cross-sectional area of the discharge opening include U.S. Pat. Nos. 5,338,234, 5,658,176, 5,679,035, 5,683,276, 5,700,170, and 5,863,229. Unfortunately, these conventional variable venturis suffer from drawbacks.